In this sentence, the noun "καφές" is in accussative case (that's why it is καφέ and not καφές). The same applies for its adjectives. so μαύρος becomes μαύρο (in accussative)
Why was the sigma removed from the accusative? What normally happens to accusative words?
Yes, there is noun declination in Greek and accusative loses the -ς in masculine nouns.
Now I'm curious. How is this said in Greek: "Blacker than a moonless night, hotter and more bitter than hell itself... that is coffee."
Πιο μαύρος κι απο αφέγγαρη νύχτα, πιο καυτός και πιο πικρός απο την ίδια την κόλαση... Αυτός είναι ο καφές.